class Account
  def deposit(amount)
    @balance_variable=amount
  end
  def balance_function
    @balance_variable
  end
end

class Teller
  def initialize(cash_slot)
    @cash_slot = cash_slot
  end
  def withdraw_from(account, amount)
    @cash_slot.dispense(amount)
  end
end

class CashSlot
  def contents
    @contents or raise("I'm empty!")
  end
  def dispense(amount)
    @contents = amount
  end
end